I had a bowl of Honey Nut Cheerios recently. It’d been awhile. Regular Cheerios are more my thing. But sometimes I finish my box faster than my kids do and find myself straying to their side of the cupboard.

Honey Nut is America’s best-selling breakfast cereal, and by a comfortable margin. Roughly 151 million boxes and other containers of various sizes were sold over the past year, well ahead of the second best-selling breakfast cereal, Frosted Flakes, according to IRI, a Chicago based market research firm.

I had no idea. The only thing I could think about when I ate it again for the first time in years was how incredibly sweet it is. I looked at the back of the box and could see why. Three of the top six ingredients are sweeteners: sugar, brown sugar and honey.

Previously, I assumed Honey Nut Cheerios was a slightly sweeter Cheerios, but you learn things when you finally get around to reading the back of the box. It actually has about nine times as much sugar as plain Cheerios, per serving. An of a number of popular cereals — a report that linked sugary cereals to the “nation’s childhood obesity epidemic” — put Honey Nut Cheerios’s sugar content second only to Fruity Pebbles. The same group found that one cup of the cereal had more sugar than three Chips Ahoy! cookies.

I asked General Mills about this, over a period of several days. They did not come to the phone, but responded with a series of communiqués.

“You mentioned that three of the top six ingredients in Honey Nut Cheerios are sugar, brown sugar and honey,” Mike Siemienas, a spokesman for the company, wrote in a statement. “What you didn’t mention is that the number one ingredient is oats. To be so singularly focused on one ingredient — sugar — is irresponsible and doesn’t help consumers look at the total nutrition offered.”

Honey Nut Cheerios debuted in 1979. The cereal’s forebear, Cheerios, originally called Cheerioats, was born in 1941 and has a more favorable nutrition profile. The plain version is low in sugar, “and you’re getting some oat fiber that can help lower your cholesterol modestly,” said Bonnie Liebman, director of nutrition at the Center for Science in the Public Interest.

Unfortunately, that makes the contrast with its spinoffs difficult. Those include far more sugary options like Pumpkin Spice Cheerios and Apple Cinnamon Cheerios. The newest offering, Chocolate Peanut Butter Cheerios, has what looks like sugar flakes on it, but it actually has a lower sugar content than Honey Nut Cheerios.

“The company has capitalized on the good name of the original,” Ms. Liebman said. “We’ve been saying for years that Honey Nut Cheerios has more sugar than honey and more salt than nuts, in fact it’s got no nuts at all, it’s just got almond flavor.”

The center has quarreled with General Mills before. It sued the company over health claims made about another Cheerios offshoot, Cheerios Protein.

General Mills refers to cereals like Honey Nut Cheerios as “presweetened” — putting the sugar, brown sugar and honey into your cereal so you don’t have to. Back in 2009, the company made news by announcing an initiative had already been underway to drop the sugar figure into single digits in such cereals marketed to children. In the last decade or so, Honey Nut Cheerios has fallen to nine grams of sugars per serving, from 11. Or so it seems.

I undertook a review of highly sensitive corporate documents. And by that, I mean I looked at pictures of old cereal boxes on eBay, because apparently there’s a market for vintage, flattened cereal boxes. I found a box from 2003 that showed the serving size of Honey Nut Cheerios to be one cup, weighing 30 grams and having 11 grams of sugars. Today, a serving is three-quarters of a cup, and just 28 grams, with nine grams of sugars.

The serving size of regular Cheerios remains one cup. If Honey Nut Cheerios still had a one cup serving size, the sugar content would be in the double digits.

General Mills said little about what had happened, or when.

“There are several reasons that changes in serving size may occur including recipe improvements that may change the density of the cereal or regulation changes,” Mr. Siemienas wrote.

Serving sizes are regulated by the Food and Drug Administration, and can change if a cereal’s density changes. It appears that at some point in the last 10 or 15 years, General Mills tweaked the ingredients of Honey Nut Cheerios in a way that lowered the overall weight per serving, which had the effect of helping lower the stated sugar content. It has not been all smoke and mirrors; the percentage of sugar has decreased slightly as well, though rounding makes it difficult to say by how much.

“They’ve tested and tested and tested to try to find out how much sugar they can take out without it affecting sales,” said Marion Nestle, an emerita professor of nutrition and food studies at New York University. “They must surely be at the cut point.”

Speaking for myself, I eat a lot more cereal than three-quarters of a cup. I poured that amount into a bowl. It looked sad. And small. It was an appetizer’s worth of Cheerios.

Don’t worry, General Mills. I’ll continue to eat Cheerios. But I had to have a difficult conversation with a 12-year-old recently. I broke the news that Honey Nut would not be returning to the cupboard. We’d be sticking with regular Cheerios.